This invention deals with a novel use of silver-copper-palladium brazing alloys.
Such brazing alloys are commercially available, cf. xe2x80x9cWelding Journal, October 1990, pages 31 to 34, which describes, among many other brazing alloys whose ability to wet 316L steel is investigated, a 68Ag-27Cu-5Pd brazing alloy designated as xe2x80x9cPalcusil 5xe2x80x9d, a 58Ag-32Cu-10Pd brazing alloy designated as xe2x80x9cPalcusil 10xe2x80x9d, a 65Ag-20Cu-15Pd brazing alloy designated as xe2x80x9cPalcusil 15xe2x80x9d, and a 54Ag-21Cu-25Pd brazing alloy designated as xe2x80x9cPalcusil 25xe2x80x9d.
Since these silver-copper-palladium brazing alloys properly wet stainless steel, they can be used for brazing components made of this material. It is also possible, however, to braze components of titanium with these silver-copper-palladium brazing alloys.
When examining how to braze a component of titanium to a component of stainless steel, i.e. without first having to apply an intermediate layer of another metal to the steel, for instance nickel to 304L steel, cf. xe2x80x9cWelding Journal, May 1991, page 112, the inventor first noted only that, if flat surfaces of the two components are brazed, the joint is brittle after having cooled down.
This is due to the rather different coefficients of thermal expansion of these two materials; the expansion coefficient of steel is quite a bit greater than that of titanium. Surprisingly, however, silver-copper-palladium brazing alloys, which have hitherto been offered only for the brazing of components of the same material, are also very well suited for brazing titanium to stainless steel if, according to one feature of the invention, the second component, i.e., the component of stainless steel, clasps the first component, i.e., the component of titanium, tightly, so that the cold joint is under constant compressive stress.
Accordingly, a first variant of the invention consists in the use of silver-copper-palladium brazing alloys for brazing a first component of titanium to a second component of stainless steel which clasps the first component tightly.
A second variant of the invention provides a method for forming a compound arrangement by brazing a first component of titanium to a second component of stainless steel which clasps the first component tightly, using silver-copper-palladium brazing alloys, wherein
the first component of titanium is provided with a cylindrical first end
which has a smaller outside diameter than an adjacent main portion
whose external surface is, at least in part, a first surface to be brazed;
the second component is a cylindrical steel sleeve
whose inside diameter is equal to the outside diameter of the main portion of the first component and
whose internal surface is, at least in part, a second surface to be brazed;
a silver-copper-palladium brazing alloy is placed around the first end of the first component;
the steel sleeve is slipped over the main portion of the first component; and
the first and second components and the silver-copper-palladium brazing alloy are heated in a vacuum or an inert gas until the silver-copper-palladium brazing alloy melts and wets the surfaces to be brazed, and are then allowed to cool down;
whereby the compound arrangement is formed.
A first development of the second variant of the invention provides a method wherein
the steel sleeve has an end projecting beyond the first end of the first component of titanium;
the first component has a tapped blind hole at the first end;
a tube of stainless steel which has an outside diameter equal to the inside diameter of the steel sleeve is provided at a first end with an external thread fitting the thread of the tapped blind hole; and
the projecting end of the steel sleeve is brazed to the tube.
A second development of the second variant of the invention, which can also be used together with the first development, provides a method wherein the main portion of the first component of titanium is provided with a collar remote from the first end, said collar being covered by and serving as a stop for the steel sleeve.
A third development of the second variant of the invention, which can also be used with the first development and/or the second development, provides a method wherein
the first component of titanium is provided with an axial bore whose diameter is equal to the inside diameter of the tube of stainless steel;
a titanium tube whose outside diameter is virtually equal to the inside diameter of the tube is inserted into the tube and into the axial bore; and
the titanium tube is electrically welded to the first component in an inert-gas atmosphere.
In a preferred embodiment of the first or second variant of the invention, which can also be used with the above developments, a composition of 86.5 wt. % silver, 26.5 wt. % copper, and 5 wt. % palladium is used which is as free of residues as possible.